Conventionally, in an image forming device, a paper feeding unit is generally provided to separate and send out paper sheets one by one from a paper sheet cassette containing the paper sheets to an image forming unit. While a paper feeding unit of a pad retarding type (FR type), or of a friction retarding type (FRR type) or the like is conventionally put into practical use, one in the FRR type is often employed because of high reliability in preventing duplicate feed of the paper sheets.
The paper feeding unit in the FRR type has: a picking-up roller which comes into contact with the uppermost part of paper sheets contained in a paper sheet cassette and draws the paper sheet out of the paper sheet cassette with friction force; a feeding roller which sends out the paper sheet drawn out to a paper sheet conveying path; and a retard roller which is brought into press contact with the feeding roller and is stopped or given rotational driving force in the reverse direction to the paper sheet conveying direction via a torque limiter.
In the paper feeding unit, when only one paper sheet passes through the press contact part of the feeding roller and the retard roller, slippage of the torque limiter allows the retard roller to be driven with the feeding roller and to rotate in the paper sheet conveying direction, and the feeding roller sends out the paper sheet in the conveying direction. On the other hand, when two or more paper sheets are to pass through the press contact part, the retard roller stops or rotates in the reverse direction to the paper sheet conveying direction and returns the paper sheet in contact with the retard roller in the direction to the paper sheet cassette, and the paper sheet in contact with the feeding roller is sent out in the conveying direction. Thus, duplicate feed of paper sheets is prevented.
These paper feed rollers generally have a shaft body and an elastic layer formed on the outer circumference of the shaft body. As a material composing the elastic layer, there is conventionally used EPDM, thermosetting polyurethane or the like. Thermosetting polyurethane is often employed since it has lower electric resistance and sticking of paper powder due to static electricity can be easily suppressed in comparison with EPDM. It should be noted that since thermosetting polyurethane itself has high hardness and a low coefficient of friction, a plasticizer is generally blended to secure a predetermined coefficient of friction (for example, refer to Patent Document 1).